1. Field of the Invention
The present invention relates to a rotary electric machine and, particularly, an armature having an integrated commutator at an end thereof.
2. Description of the Related Art
U.S. Pat. No. 5,739,617 or its corresponding Japanese Patent Application JP-A-9-51645 discloses an armature of a rotary electric machine. The armature has an integrated commutator at an end of an armature coil. Upper-layer conductor segments and a generally disk-like insulation spacer, which is made of a hard resinous material, are fitted to each other on the end of the armature adjacent a commutator so that axial position of the upper-layer conductor segments can be fixed. Therefore, the commutator surface in contact with the brush can be maintained flat.
Although the commutator surface of the armature can be maintained flat to a certain extent by the insulation spacer, there is still considerable unevenness on the surface. For example, the commutator surface of an armature of a small starter motor for a vehicle, which has 50 mm in diameter and 30 mm in length, had about 20 xcexcm difference in height thereon after a starting operation was repeated. It is noted that the difference in height of the commutator surface was measured after the starter motor had been tested and disassembled. That is, more difference in height should have been measured if the difference was measured while the starter motor was operating.
If there is a certain large difference in height on the commutator surface 31a, the brush may wear away. As a result, the lifetime of the brush shortens, and electric resistance loss between the commutator and the brush increases.
Therefore, the present invention is to provide an armature that has more flat commutator surface.
If upper layer conductor segments are heated in a short time and the temperature thereof rises sharply, difference in thermal expansion are generated between the upper-layer conductor segment and the armature core. In addition, the following fact was found: the upper-layer conductor segments, which form the commutator, expand and slide on the armature core due to the thermal expansion. If the upper-layer conductor segments expand uniformly, there is no difference in height on the commutator surface. The difference in height on the commutator surface is generated if the upper-layer conductor segments expand diversely, thereby causing the upper-layer conductor segments to shift diversely. Because the pressure is uniformly applied on the upper-layer conductor segments to retain in the slots of the armature core in the axial direction, positions of the upper-layer conductor segments when thermally expanding may be different from positions there of when thermally contracting. In other words, each upper-layer conductor segment moves like an earthworm on the armature core every time it thermally expands and contracts, or shifts as a result of repeated random motions in a certain distance.
According to a main feature of the invention in order to prevent the above shifts, an armature core has an anchoring portion, disposed near the commutator, for anchoring a part of each the conductor segment more strongly than the rest thereof.
Because a part of each conductor segment is tightly fixed to the anchoring portion, there is no shift of each conductor segment at the anchoring portion relative to the armature core even if each conductor segment thermally expands or contracts. Even if the conductor segments repeats thermal expansion and thermal contraction while the rotary electric machine is operating, the anchored parts of the conductor segments relative to the armature core do not shift. Accordingly, even if the rotary electric machine operates for a long time, there is no height difference on the commutator segment of the armature.
As a result, the lifetime of the brushes is increased and the service cost thereof is reduced. In addition, the connection resistance between the commutator and the brushes is reduced, thereby improving the performance of the rotary electric machine.
Each anchored part of the conductor segment is preferably fixed to the armature core at a distance from the commutator as short as possible. If the distance is not short enough, there is some possibility of the height difference on the commutator surface due to irregular friction and others generated between the armature core and the conductor segments. However, it may be disposed at a distance less than a half of the length of the armature core from the end thereof adjacent to the commutator.
On the other hand, the anchoring portion may be disposed at a space corresponding to one of the laminated sheets of the armature core. If the anchoring portion is disposed at the extreme end of the armature core adjacent to the commutator, there is some possibility of inconvenience. For example, if the anchoring portion is formed by pressing the outer periphery of the armature core, insulation paper sheets that insulate the conductor segments may be broken because a large centrifugal force of the commutator is applied to the anchoring portion. In order to avoid the above problem, the anchoring portion is formed at not the extreme end of the armature core but at least a pace of the laminated sheet from the end.
The anchoring portion can be formed very easily by a roller. The roller is pressed against the outer periphery of the armature core while the armature is being turned.
Preferably, the pressed portion has an outside diameter 0.08%-0.6% less than the outside diameter of other portion of the armature core. Therefore, the pressed portion can be formed without severe accuracy of the size, so that the production cost of the armature can be maintained at a low level.
If the outside diameter of the pressed portion is not 0.08%-0.6% less than the outside diameter of other portions of the armature core, the depth of the pressing is not sufficient. In other words, the pressed depth is so small that the pressed portion is not so strong to provide the pressing force at the pressed portion. If the outside diameter of the pressed portion is 0.6% smaller than the other portions of the armature core, the pressed portion sinks too deep, and insulation paper sheets are broken at the pressed portion.
Because the pressed portion is formed at the step of applying a stepped punch on the outer periphery of the armature core that is the last step in manufacturing process of the armature, the pressed portion can be formed easily at a low cost. Almost the all parts except the spacer are well known and almost all the steps except the pressing step for forming the pressed portion 22 are well known. As a result, the production cost does not increase very much.